Color thermal printers may be of the single printhead variety or may be of a multiple printhead variety. In a single printhead color thermal printer, the color dyes, yellow, magenta and cyan, are applied to the receiver one at a time so that three applications are required to obtain a color print. A multiple printhead color thermal printer has the advantage of simultaneously applying dye to three separate areas of the receiver thereby printing three images at one time. A multiple printhead thermal printer prints all required color separations in a continuous motion by transporting the receiver sheet past a series of printheads.
A problem with multihead color thermal printing is the difficulty of properly aligning the color separations on the receiver to give crisp, high quality images. Even when the printheads are accurately positioned relative to the print drum or to the receiver pathway, there still exists the possibility for poor registration which deteriorates print quality. There is a possibility for misregistration in the direction of travel of the receiver because the receiver may stretch or become misaligned on the drum. Also, when the receiver web is misaligned or skewed on the drum, the sideways orientation is affected. It will be appreciated that it would be highly desirable to have a multiple printhead color thermal printer which produces a high quality color print with excellent registration.
Although the printheads may be accurately positioned and precisely aligned relative to the print drum or to the receiver pathway, mechanical adjustments are impractical, if not impossible, during operation of a thermal printer. Without the ability to adjust the print process electronically, accurate printhead position is required during operation, both along the receiver path and side to side across the receiver path to ensure exact print registration between images from each printhead. Accordingly, it will be appreciated that it would be highly desirable to have a thermal printer which employs electronic print devices and circuitry to ensure exact registration between images from each printhead.